1. Field of the Invention
The present invention is concerned with detecting and identifying naturally radioactive materials present in an earth formation of interest and, more particularly, a method and apparatus for analyzing a detected spectrum of naturally emitted gamma radiation to determine the presence of a selected number of naturally radioactive materials.
2. Description of the Prior Art
The potential usefulness of being able to measure individually the contributions of the primary sources of natural radiation has been recognized for many years. Such information can, to cite a few instances, be useful in well-to-well correlation, in the study and evaluation of sedimentary basins, in the quantitative measurement of radioactive mineral deposits in a formation, and in conjunction with other techniques, such as neutron activation, to obtain in situ elemental analysis.
An early technique for the location of mineral deposits that finds general utility in distinguishing between geological formations and in locating discontinuities in the earth's crust is described in U.S. Pat. No. 2,749,446, issued June 5, 1956 to Herzog. Simply put, Herzog discloses that it is possible to distinguish between geological formations by recording the detected gamma ray spectrum. The recorded spectrum, according to Herzog, may either be an instantaneous record giving frequency and amplitude information or a time averaged value, although the latter format could prevent the observer from distinguishing between formations. Little constituent identification may be accomplished, however, in formations having more than one radioactive material.
In a later form of natural gamma ray well logging, the counting rates of gamma rays detected in three channels or spectra centered on the 1.46 MeV potassium, the 1.76 MeV uranium, and the 2.62 MeV thorium energies are collected to determine the count contributions and thereby the relative presence of the individual materials to the total count rate. The individual channel or spectrum widths were set at .+-. 10 percent of the energy value for the particular peak.
Such techniques, while an improvement over the prior methods, still tend to have a rather low statistical accuracy and a relatively high systematic error in evaluating the potassium, uranium and thorium contents of the formation. The detected gamma ray spectrum at any single energy includes a contribution from all of the radioactive materials in the formation, and to rely on a single relatively narrow peak for information as to a selected material, results in ignoring a significant amount of the total information available in the detected gamma ray spectrum.
Another reported technique uses detected gamma ray energy bands that are centered about selected energy peaks. Experiments were performed on specially prepared earth and building material samples taken around a location where an overall dose rate had been measured, and a low-energy band/high-energy band method involving two groupings of three energy ranges was used to determine constituent radioactive material contributions. While a further improvement, statistical accuracy is still sacrificed, however, in that portions of the detected gamma ray spectrum containing relevant information are ignored.